josh bloch charlie garrod - carnegie mellon school of...

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SchoolofComputerScience

PrinciplesofSo3wareConstruc9on: Objects,Design,andConcurrencyDesigning(sub-)systemsAformaldesignprocess,part2JoshBloch CharlieGarrod

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Administrivia

•  MidtermexamThursday–  ReviewsessionWednesday,September28th,7-9pm,HHB103

•  Homework4–  Threeparts,partAdueOct6th–  Designreviewmee9ngsnextweek

•  Collabora9onpolicy…

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KeyconceptsfromlastThursday…

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Sequenceandcommunica9ondiagrams

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Designprinciples

•  Lowcoupling•  Lowrepresenta9onalgap•  Highcohesion

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Buildingadomainmodelforalibrarysystem

Apubliclibrarytypicallystoresacollec9onofbooks,movies,orotherlibraryitemsavailabletobeborrowedbypeoplelivinginacommunity.Eachlibrarymembertypicallyhasalibraryaccountandalibrarycardwiththeaccount’sIDnumber,whichshecanusetoiden9fyherselftothelibrary.Amember’slibraryaccountrecordswhichitemsthememberhasborrowedandtheduedateforeachborroweditem.Eachtypeofitemhasadefaultrentalperiod,whichdeterminestheitem’sduedatewhentheitemisborrowed.Ifamemberreturnsanitema3ertheitem’sduedate,thememberowesalatefeespecificforthatitem,anamountofmoneyrecordedinthemember’slibraryaccount.

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Onedomainmodelforthelibrarysystem

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Notesonthelibrarydomainmodel

•  Allconceptsareaccessibletoanon-programmer•  TheUMLissomewhatinformal

–  Rela9onshipsareo3endescribedwithwords•  Real-world"is-a"rela9onshipsareappropriateforadomainmodel•  Real-wordabstrac9onsareappropriateforadomainmodel•  Itera9onisimportant

–  Thisexampleisafirstdra3.Someterms(e.g.Itemvs.LibraryItem,Accountvs.LibraryAccount)wouldlikelyberevisedinarealdesign.

•  Aggregatetypesareusuallymodeledasclasses•  Primi9vetypes(numbers,strings)areusuallymodeledasadributes

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BuildadomainmodelforMonopoly

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BuildadomainmodelforMonopoly

Monopolyisagameinwhicheachplayerhasapiecethatmovesaroundagameboard,withthepiece’schangeinloca9ondeterminedbyrollingapairofdice.Thegameboardconsistsofasetofproper9es(ini9allyownedbyabank)thatmaybepurchasedbytheplayers.Whenapiecelandsonapropertythatisnotowned,theplayermayusemoneytobuythepropertyfromthebankforthatproperty’sprice.Ifaplayerlandsonapropertyshealreadyowns,shemaybuildhousesandhotelsontheproperty;eachhouseandhotelcostssomepricespecificfortheproperty.Whenaplayer’spiecelandsonapropertyownedbyanotherplayer,theownercollectsmoney(rent)fromtheplayerwhosepiecelandedontheproperty;therentdependsonthenumberofhousesandhotelsbuiltontheproperty.Thegameisplayedun9lonlyoneremainingplayerhasmoneyandproperty,withalltheotherplayersbeingbankrupt.

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Ar9factsofthisdesignprocess

•  Model/diagramtheproblem,defineobjects–  Domainmodel(a.k.a.conceptualmodel)

•  Definesystembehaviors–  Systemsequencediagram–  Systembehavioralcontracts

•  Assignobjectresponsibili9es,defineinterac9ons–  Objectinterac9ondiagrams

•  Model/diagramapoten9alsolu9on–  Objectmodel

Understandingtheproblem

Definingasolu9on

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Ar9factsofthisdesignprocess

•  Model/diagramtheproblem,defineobjects–  Domainmodel(a.k.a.conceptualmodel)

•  Definesystembehaviors–  Systemsequencediagram–  Systembehavioralcontracts

•  Assignobjectresponsibili9es,defineinterac9ons–  Objectinterac9ondiagrams

•  Model/diagramapoten9alsolu9on–  Objectmodel

Today

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Understandingsystembehaviorwithsequencediagrams

•  Asystemsequencediagramisamodelthatshows,foronescenarioofuse,thesequenceofeventsthatoccuronthesystem’sboundary

•  Designgoal:Iden9fyanddefinetheinterfaceofthesystem–  Twocomponents:Auserandtheoverallsystem

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Understandingsystembehaviorwithsequencediagrams

•  Asystemsequencediagramisamodelthatshows,foronescenarioofuse,thesequenceofeventsthatoccuronthesystem’sboundary

•  Designgoal:Iden9fyanddefinetheinterfaceofthesystem–  Twocomponents:Auserandtheoverallsystem

•  Input:Domaindescrip9onandoneusecase•  Output:Asequencediagramofsystem-levelopera9ons

–  Includeonlydomain-levelconceptsandopera9ons

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Onesequencediagramforthelibrarysystem

Usecasescenario:Alibrarymembershouldbeabletouseherlibrarycardtologinatalibrarysystemkioskandborrowabook.A3erconfirmingthatthememberhasnounpaidlatefees,thelibrarysystemshoulddeterminethebook’sduedatebyaddingitsloanperiodtothecurrentday,andrecordthebookanditsduedateasaborrowediteminthemember’slibraryaccount.

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Onesequencediagramforthelibrarysystem

Usecasescenario:Alibrarymembershouldbeabletouseherlibrarycardtologinatalibrarysystemkioskandborrowabook.A3erconfirmingthatthememberhasnounpaidlatefees,thelibrarysystemshoulddeterminethebook’sduedatebyaddingitsloanperiodtothecurrentday,andrecordthebookanditsduedateasaborrowediteminthemember’slibraryaccount.

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BuildonesystemsequencediagramforMonopoly

Usecasescenario:Whenaplayerlandsonanunownedpropertyandhasenoughmoneytobuytheproperty,sheshouldbeabletobuythepropertyfortheproperty’sprice.Thepropertyshouldnolongerbepurchasablefromthebankbyotherplayers,andmoneyshouldbemovedfromtheplayertothebank.

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Formalizesystembehaviorwithbehavioralcontracts

•  Asystembehavioralcontractdescribesthepre-condi9onsandpost-condi9onsforsomeopera9oniden9fiedinthesystemsequencediagrams–  System-leveltextualspecifica9ons,likeso3warespecifica9ons

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Asystembehavioralcontractforthelibrarysystem

Opera9on: borrow(item)Pre-condi9ons: Librarymemberhasalreadyloggedintothesystem.

Itemisnotcurrentlyborrowedbyanothermember.Post-condi9ons: Logged-inmember'saccountrecordsthenewly-borrowed

item,orthememberiswarnedshehasanoutstandinglatefee. Thenewly-borroweditemcontainsafutureduedate, computedastheitem'srentalperiodplusthecurrentdate.

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Dis9nguishingdomainvs.implementa9onconcepts

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Dis9nguishingdomainvs.implementa9onconcepts

•  Domain-levelconcepts:–  Almostanythingwithareal-worldanalogue

•  Implementa9on-levelconcepts:–  Implementa9on-likemethodnames–  Programmingtypes–  Visibilitymodifiers–  Helpermethodsorclasses–  Ar9factsofdesignpaderns

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Summary:Understandingtheproblemdomain

•  Knowyourtoolstobuilddomain-levelrepresenta9ons–  Domainmodels–  Systemsequencediagrams–  Systembehavioralcontracts

•  Befastand(some9mes)loose–  Elideobvious(?)details–  Iterate,iterate,iterate,…

•  Getfeedbackfromdomainexperts–  Useonlydomain-levelconcepts

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Drawadomainmodelforcryptarithmsolving

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Ar9factsofourdesignprocess

•  Model/diagramtheproblem,defineobjects–  Domainmodel(a.k.a.conceptualmodel)

•  Definesystembehaviors–  Systemsequencediagram–  Systembehavioralcontracts

•  Assignobjectresponsibili9es,defineinterac9ons–  Objectinterac9ondiagrams

•  Model/diagramapoten9alsolu9on–  Objectmodel

Understandingtheproblem

Definingasolu9on

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Object-orientedprogramming

•  Programmingbasedonstructuresthatcontainbothdataandmethods

publicclassBicycle{privateintspeed;privatefinalWheelfrontWheel,rearWheel;privatefinalSeatseat;…publicBicycle(…){…}publicvoidaccelerate(){speed++;}publicintspeed(){returnspeed;}}

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Responsibilityinobject-orientedprogramming

•  Data:–  Privateorotherwiseencapsulateddata–  Dataincloselyrelatedobjects

•  Methods:–  Privateorotherwiseencapsulatedopera9ons–  Objectcrea9on,ofitselforotherobjects–  Ini9a9ngac9onsinotherobjects–  Coordina9ngac9vi9esamongobjects

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Usinginterac9ondiagramstoassignobjectresponsibility

•  Foragivensystem-levelopera9on,createanobjectinterac9ondiagramattheimplementa6on-levelofabstrac9on–  Implementa9on-levelconcepts:

•  Implementa9on-likemethodnames•  Programmingtypes•  Helpermethodsorclasses•  Ar9factsofdesignpaderns

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Exampleinterac9ondiagram#1

Usecasescenario:Alibrarymembershouldbeabletouseherlibrarycardtologinatalibrarysystemkioskand…

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Exampleinterac9ondiagram#2

Usecasescenario:…andborrowabook.A3erconfirmingthatthememberhasnounpaidlatefees,thelibrarysystemshoulddeterminethebook’sduedatebyaddingitsloanperiodtothecurrentday,andrecordthebookanditsduedateasaborrowediteminthemember’slibraryaccount.

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Interac9ondiagramshelpevaluatedesignalterna9ves

Createtwopossibleinterac9ondiagrams:1.  Solvingacryptarithm,assumingthatthecryptarithmclasshas

responsibilityforsolvingitself2.  Solvingacryptarithm,assumingthatthemainmethod(oradelegated

methodorclass)hasresponsibilityforsolvingthecryptarithm

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Heuris9csforresponsibilityassignment

•  Controllerheuris9c•  Informa9onexpertheuris9c•  Creatorheuris9c

Goals

Heuristics Patterns

Principles

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Thecontrollerheuris9c

•  Assignresponsibilityforallsystem-levelbehaviorstoasinglesystem-levelobjectthatcoordinatesanddelegatesworktootherobjects–  Alsoconsiderspecificsub-controllersforcomplexuse-casescenarios

•  Designprocess:Extractinterfacefromsystemsequencediagrams–  Keyprinciples:Lowrepresenta9onalgapandhighcohesion

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Informa9onexpertheuris9c

•  Assignresponsibilitytotheclassthathastheinforma9onneededtofulfilltheresponsibility–  Ini9aliza9on,transforma9on,andviewsofprivatedata–  Crea9onofcloselyrelatedorderivedobjects

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Responsibilityinobject-orientedprogramming

•  Data:–  Privateorotherwiseencapsulateddata–  Dataincloselyrelatedobjects

•  Methods:–  Privateorotherwiseencapsulatedopera9ons–  Objectcrea9on,ofitselforotherobjects–  Ini9a9ngac9onsinotherobjects–  Coordina9ngac9vi9esamongobjects

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Informa9onexpertheuris9c

•  Assignresponsibilitytotheclassthathastheinforma9onneededtofulfilltheresponsibility–  Ini9aliza9on,transforma9on,andviewsofprivatedata–  Crea9onofcloselyrelatedorderivedobjects

•  Designprocess:Assignmentfromdomainmodel–  Keyprinciples:Lowrepresenta9onalgapandlowcoupling

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Usetheinforma9onexpertheuris9c

•  InHomework3,whatobjectshouldhavetheresponsibilitytosolveacryptarithm?

•  Whatistherelevantinforma9on?

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Usetheinforma9onexpertheuris9c

•  InHomework3,whatobjectshouldhavetheresponsibilitytosolveacryptarithm?

•  Whatistherelevantinforma9on?–  Whoknowsthe#ofdigits(e.g.base10)inthecryptarithm?–  Whoknowstheledersofthecryptarithm?–  Whocanevaluatethecryptarithmexpressionstocheckforequality?

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Anotherdesignprinciple:Minimizeconceptualweight

•  Labeltheconceptsforaproposedobject–  Relatedtorepresenta9onalgapandcohesion

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Creatorheuris9c:WhocreatesanobjectFoo?

•  Assignresponsibilityofcrea9nganobjectFootoaclassthat:–  Hasthedatanecessaryforini9alizinginstancesofFoo–  Contains,aggregates,orrecordsinstancesofFoo–  CloselyusesormanipulatesinstancesofFoo

•  Designprocess:Extractfromdomainmodel,interac9ondiagrams–  Keyprinciples:Lowcouplingandlowrepresenta9onalgap

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Usethecreatorheuris9c

•  InHomework3,whatobjectshouldhavetheresponsibilityforcrea9ngthepermuta9ongenerator?

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Object-levelar9factsofthisdesignprocess

•  Objectinterac9ondiagramsaddmethodstoobjects–  Caninferaddi9onaldataresponsibili9es–  Caninferaddi9onaldatatypesandarchitecturalpaderns

•  Objectmodelaggregatesimportantdesigndecisions–  Isanimplementa9onguide

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Crea9nganobjectmodel

•  Extractdata,methodnames,andtypesfrominterac9ondiagrams–  Includeimplementa9ondetailssuchasvisibili9es

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Createanobjectmodelforyourcryptarithmsolver

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Summary:

•  Object-levelinterac9ondiagramsandobjectmodelsystema9callyguidethedesignprocess–  Convertdomainmodel,systemsequencediagram,andcontractsto

object-levelresponsibili9es

•  Useheuris9cstoguide,butnotdefine,designdecisions•  Iterate,iterate,iterate…